1. Field of the Invention
The invention relates generally to light guide plates used in back light systems of liquid crystal display devices and, more particularly, to a light guide plate having high-density dots.
2. Discussion of Related Art
Back light systems are used in liquid crystal display devices for converting linear light sources such as cold cathode ray tubes, or point light sources, such as light emitting diodes, into area light sources having high uniformity and brightness.
A conventional back light system includes a light source, a light guide plate, a reflection plate, a diffusion plate and a prism sheet. The light source can be located beside one end or beside two opposite ends of the light guide plate and is used to emit incident light beams into the light guide plate. The light guide plate is used to lead travel of the incident light beams therein and ensure that most of the incident light beams can be emitted from an emission surface thereof. The reflection plate is located below a bottom surface of the light guide plate and is used to reflect some of the incident light beams that are emitted from the bottom surface into the light guide plate. This reflection enhances the utilization ratio of the incident light beams. The diffusion plate and the prism sheet are located on the emission surface of the light guide plate, in turn, and are used to improve uniformity of the emitted light beams.
As shown in FIG. 10, a conventional surface light source device includes a light guide plate 1, a line light source 2, an end edge reflection layer 4, a light diffusion layer 6, a white back face reflection layer 5, and a curved reflection plate 7. The line light source 2 is positioned on a first end edge of the light guide plate 1. The end edge reflection layer 4 is provided on a second end edge of the light guide plate 1, and the light diffusion layer 6 is provided upon a light emitting surface of the light guide plate 1. The white back face reflection layer 5 is provided on a back surface of the light guide plate 1. The curved reflection plate 7 is further provided to enclose the line light source 2 so as to effectively utilize light beams emitted by the line light source 2.
Furthermore, a light diffusion/transmission section 3 is provided on the back surface of the light guide plate 1. The light diffusion/transmission section 3 is formed by means, for example, of gravure printing, offset printing, and/or screen printing or transfer and, as formed, includes a plurality of dots. The dots can have arbitrary shapes such as round, square or chain dot-shapes, and are used to break up what would otherwise be a total reflection condition of the incident light beams. This light diffusion ensures that most of the light beams can pass through the light emitting surface of the light guide plate 1.
Referring to FIG. 11, a distribution of the dots on the back surface of the light guide 1 is shown. In a region from the first end edge of the light guide plate 1 to a position at which the surface emission luminance of the light guide plate 1 is lowest, the ratio of dot area of the light diffusion/transmission section 3 to the whole area of the back surface of the light guide plate 1 gradually increases with an increase in distance from the first end edge along a first direction. The first direction is perpendicular to the end edges of the light guide plate 1 and parallel to the side edges of the light guide plate 1. In a region from the position at which the surface emission luminance of the light guide plate 1 is lowest to the second end edge of the light guide plate 1, the ratio is constant along the first direction. The ratio of the dot area of the light diffusion/transmission section 3 to the whole area of the back surface of the light guide plate 1 is made constant along a second direction perpendicular to the first direction.
The light diffusion/transmission section 3 can enhance, to a certain extent, the uniformity of the emitted light beams from the light guide plate 1. However, two corners of the first end edge of the light guide plate 1 are electroshock areas of the line light source 2, and the brightness of the emitted light beams at the two corners is relatively low. Furthermore, a clearance between adjacent dots of the light diffusion/transmission section 3 is relatively big. That is to say, the distribution density of the dots is relatively small, and, as such, the light diffusion/transmission section 3 can't disperse light beams as effectively as would be desired. Thus, it is difficult to achieve entire or even substantial uniformity of the emitted light beams from the whole area of the light guide plate 1.
Furthermore, clearances between adjacent columns of dots are straight and tend to produce bright lines in use. Thus, the light guide plate 1 can't provide an optimal display quality.
What is needed, therefore, is a light guide plate that can provide emission of light beams with good uniformity.
What is also needed is a light guide plate that can provide improved display quality.